Re: Where the genetic variation found within a population comes from?

Our bodies are made up of millions of cells. In one very general sense
these cells can be classified into two types: somatic cells, which are
diploid (contain two copies of every chromosome) and make up the
overwhelming majority of our tissues and organs, and germ-line cells, which
are haploid (contain one copy of each chromosome) and are our gametes.
Gametes are the cells that come together to form offspring in all
sexually-reproducing organisms. This results in life occuring as a cycle of
a single diploid cell reproducing to form a multicellular organism, which
will in turn produce haploid cells which will then be used to create the
next generation. In the process of going from a single-celled zygote to a
fecund multicellular adult the entire genome must be reproduced once for
every cellular division. It is during this process of genome replication
where errors can occur (ignoring the action of environmental agents to
change DNA sequences). In the nucleus of the cell are proteins that exist
to proofread the fidelity of the new DNA strands as they are created.
Generally these proteins do a very good job of ensuring that the new copies
match the original. However, they are not perfect and discrepencies
(mutations) occur. On average, there is one mutation passed on to an
offspring in every generation. If these mutations fall in regions of the
genome that do not have an important function, or when they fall in
functional regions they do not detrementally affect that function, then
they have a reasonably high probability of being passed on to further
generations. It is the accumulation of these mutations over time that lead
to the genetic variability seen in populations. The infrequency of mutation
is why it is difficult to find differences between closely related
individuals (it's like searching for a needle in a haystack). It is also
this general level of correlation of amount of difference at the DNA level
with previously perceived relationships among groups of organisms that is
seen as evidence supporting Darwin's theory of evolution by descent through
natural selection. More details about DNA replication, proofreading, and
mechanisms of mutation can be found in molecular biology textbooks, such as
those authored by Suzuki, et al. and Lewin.